The main components of a drivetrain are a drive aggregate and a transmission. The transmission converts rotation speeds and torques so as to deliver traction force supplied by the drive aggregate to a drive output of the drivetrain. The present invention concerns a method for operating a drivetrain comprising a drive aggregate which includes at least one electric machine, and preferably a transmission connected between the drive aggregate and a drive output. The transmission is preferably in the form of an automatic or automated manual transmission. In a purely electric vehicle the transmission can if necessary be omitted.
It is already known from the prior art to operate the drivetrain of a motor vehicle in a crawling mode when a gear is engaged in an automatic or automated transmission of the drivetrain and an accelerator pedal and a brake pedal of the drivetrain are not actuated, such that in this crawling mode a crawling torque is delivered which enables the motor vehicle, operated in the crawling mode, to roll at a low speed.
From DE 10 2006 003 714 A1 a speed-regulated crawling operation mode of a drivetrain of a motor vehicle is known. During the speed-regulated crawling operation, a nominal speed value is compared with an actual speed value and on the basis of the difference between the nominal and the actual speed values a crawling torque is produced as a control output.
The method known from DE 10 2006 003 714 A1 can be used with a drivetrain in which a planetary gear set is connected between the drive aggregate, which comprises an internal combustion engine and an electric machine, and the transmission.
This method is not suitable for use with a drivetrain in which no such planetary or overlap gear system is connected between the drive aggregate and the transmission. In particular, it is unsuitable for any type of parallel hybrid drivetrain in which a torque supplied by an electric machine to the drive output is added without an overlap gear system to the torque supplied to the drive output by the internal combustion engine, and for drivetrains with purely electric drive.